Nine Problems That Need Special Attention When Using PLC Programming (Indispensable)

Have you ever encountered a power board failure? Have you encountered output and input faults? Has the program been lost?

Maybe you only encountered one PLC output point on the road for more than 10 years, and there are no other PLC hardware problems! You may have encountered problems with electronic boards after welding. . Perhaps Siemens 200 series programming port burns N many times, the output relay sticks, the transistor output short circuit and so on.

Are you very upset with these problems?

It is said that to comply with the use of the nine principles of PLC can effectively avoid, but also can avoid detours ~

Nine major considerations for PLC use:

One, PLC own fault judgment

In general, PLC is an extremely reliable device with a low failure rate. The probability of PLC hardware damage such as PLC or software running is almost zero; if PLC input point is not caused by strong power intrusion, it will not be damaged at all; PLC output relay normally open point, if not external load short circuit or design is unreasonable The load current exceeds the rated range and the contact life is also very long.

Therefore, when we look for electrical faults, we must focus on the external electrical components of the PLC. Do not always suspect PLC hardware or program problems. This is very important for quickly repairing faulty equipment and quickly recovering production. The electrical faults of the PLC control loop are not focused on the PLC itself, but are peripheral electrical components in the PLC controlled loop.

Second, the input and output (I / O) module selection

Output modules are divided into transistors, triacs, and contact types.

Transistor-type switching speed is fastest (usually 0.2ms), but the minimum load capacity, about 0.2 ~ 0.3A, 24VDC, suitable for fast switching, signal contact equipment, generally connected to the frequency conversion, DC devices and other signals, should pay attention to the transistor leakage Current impact on the load.

The advantages of thyristor type are non-contact, AC load characteristics, and small load capacity.

Relay output has AC and DC load characteristics and large load capacity. In the conventional control, the relay contact type output is generally used first. The disadvantage is that the switching speed is slow, generally about 10ms, and it is not suitable for high frequency switching applications.

Third, the grounding problem

PLC system grounding requirements are more stringent, it is best to have a separate dedicated grounding system, but also pay attention to other equipment related to PLC but also reliable grounding. When multiple circuit grounds are connected together, unexpected currents are generated, resulting in logic errors or circuit damage.

The reason for the different ground potentials is usually due to the fact that the ground points are separated too far in the physical area. When the distant devices are connected together by communication cables or sensors, the current between the cable lines and the ground is Will flow through the entire circuit, even within a short distance, the load current of a large device can also change between its ground potential, or directly generate unpredictable current through electromagnetic action.

Between the power supplies at the incorrect grounding point, there is the potential for catastrophic currents in the circuit, which can damage the equipment.

The PLC system generally uses one point grounding method. In order to improve the resistance to common mode interference, shielding floating technology can be used for analog signals, that is, the shielding layer of the signal cable is grounded at one point, the signal loop is floating, and the insulation resistance with the earth should not be less than 50 MΩ.

Fourth, eliminate the line capacitance to avoid malfunction

Capacitance exists between the wires of the cable, and qualified cables can limit this capacitance within a certain range. Even if it is a qualified cable, when the cable length exceeds a certain length, the capacitance between the lines will exceed the required value. When this cable is used for PLC input, the line capacitance may cause the PLC to malfunction. There will be many things that cannot be understood.

These phenomena are mainly manifested as follows: the wiring is correct, but the PLC has not been input; PLC should have no input, but should not have, that PLC input interfere with each other. To solve this problem, you should do:

1. Use cable cores twisted together;

2. Minimize the length of the cable used;

3. Separate the use of cables from inputs that interfere with each other;

4. Use shielded cables.

Fifth, anti-jamming treatment

The environment at the industrial site is rather harsh and there are many high and low frequency interferences. These disturbances are generally introduced into the PLC through cables connected to field devices. In addition to grounding measures, some anti-jamming measures should be taken during cable design and installation.

1. Analog signal is a small signal and is easily affected by outside interference. Double-layer shielded cable should be used; shielded cable should be used for high-speed pulse signal (such as pulse sensor, counting code disk, etc.) to prevent external interference and prevent it. High-speed pulse signal interference on low-level signals;

2. The frequency of the communication cable between the PLCs is relatively high. Generally, the cable provided by the manufacturer should be used. If the requirement is not high, the shielded twisted pair cable can be used.

3. The analog signal line and the DC signal line cannot be routed in the same slot as the AC signal line.

4. The shielded cables introduced into the control cabinet must be grounded and should not be directly connected to the equipment through the terminals;

5. AC signal, DC signal and analog signal can not share a single cable, and the power cable should be laid separately from the signal cable;

6. In the on-site maintenance, the methods to solve the interference include: using shielded cables to the interfered lines and re-laying them; adding anti-interference filter code to the program.

Sixth, mark input and output, convenient maintenance

The PLC controls a complex system. What can be seen is the upper and lower rows of staggered I/O relay terminals, the corresponding indicators, and the PLC number, just like an integrated circuit with dozens of feet. Any person who does not look at the schematic to troubleshoot a faulty device will be unable to find a solution. The speed at which faults are found will be particularly slow.

In view of this situation, we draw a table according to the electrical schematic diagram and stick it on the console or control cabinet of the equipment, indicating the electrical symbol corresponding to each PLC input and output terminal number, the Chinese name, that is similar to each IC Functional description of the foot.

With this input and output form, an electrician who understands the operation process or is familiar with the ladder diagram of the device can perform the overhaul. But for those electricians who are not familiar with the operation process and do not look at the ladder diagram, it is necessary to draw a table: PLC input and output logic function table. This table actually illustrates the logical correspondence between the input loop (trigger element, associated element) and output loop (actuator element) during most operations.

Practice has proved that if you can skillfully use the input and output correspondence table and the input and output logic function table, and repair electrical faults without drawing, you can also easily and freely.

VII. Inferring faults through program logic

Nowadays PLCs are widely used in industry. For low-end PLCs, the ladder diagram instructions are much the same. For high-end machines, such as the S7-300, many programs are compiled using language tables. The practical ladder diagram must have a Chinese symbol note, otherwise it is difficult to read. If you look at the ladder diagram before you can understand the equipment process or operation process, it seems easier.

If the electrical fault analysis is performed, it is generally applied back-checking method or back-inference method, that is, according to the input/output correspondence table, the corresponding output relay of the PLC is found from the fault point, and the logical relationship that satisfies the action is started. Experience shows that if a problem is found, the fault can be basically eliminated because there are not many faults occurring at the same time and two or more faults.

VIII. Making full and reasonable use of hardware and software resources

1. Do not participate in the control loop or the instructions that have been put in before the loop can not access the PLC; multiple instructions to control a task, you can connect them externally in the PLC before connecting an input point;

2. Make use of PLC internal functional devices as much as possible to fully invoke the intermediate state, so that the program has a complete coherence and is easy to develop. At the same time, it also reduces hardware investment and reduces costs.

3. When the conditions permit, it is better to independently output each channel to facilitate control and inspection, as well as to protect other output circuits; when an output point fails, it will only lead to the corresponding output circuit out of control;

4. If the output is a positive/inverse control load, not only must interlock from the PLC internal program, but also take measures outside the PLC to prevent the load from moving in both directions;

5. PLC emergency stop should be cut off with an external switch to ensure safety.

Nine, other matters needing attention

1. Do not connect the AC power cord to the input terminal to avoid burning the PLC. The grounding terminal should be grounded independently, not in series with other equipment grounding terminals. The cross-sectional area of the grounding wire should not be less than 2mm2.

2. The power of the auxiliary power supply is small, and it can only drive low-power devices (photoelectric sensors, etc.)

3. Some PLCs have a certain number of possession points (ie empty address terminals), do not connect the line;

4. When there is no protection in the PLC output circuit, fuses and other protection devices should be used in series in the external circuit to prevent damage due to short circuit of the load.